Communicable Diseases

OCR A Level Bio

bacteria

• prokaryotes

• can be identified by: their basic shapes (bacilli, cocci, vibrios, spirochaetes, spirilla)

• can be identified by: their cell walls (using gram staining - gram positive bacteria are stained blue-purple, gram negative bacteria are stained red)

• produce toxins that poison or damage host cells e.g breaking down cell membranes, deactivating enzymes

viruses

• basic structure consists of genetic material surrounded by proteins

• parasites (take over cell metabolism, invade host cells and reproduce rapidly to produce more viruses, destroying host cells in the process)

• 0.02 - 0.3µm in diameter

• non-living

protista

• parasites

• may need vectors to spread themselves

• take over host cells and digest and use the cell contents as they reproduce

fungi

• most feed off dead and decaying matter, but can also feed off living matter

• digest living cells and destroy them or produce toxins

• when they reproduce they produce many spores which can spread across long distances

definition of communicable disease

a disease that can be spread from one organism to another

ring rot - bacteria

• gram postive

• no cure - so have to wait min 2 years before regrowing

• affects tomatoes, potatoes and aubergines

• damages leaves, tubers and fruit (80% of the crop)

TMV (tobacco mosaic virus) - virus

• infects tobacco plants and another 150 species

• damages leaves, flowers and fruit stunting growth and reducing yields

• resistant crop strains are available but no cure

potato blight - protist

• destroys leaves, tubers and fruit

• hyphae penetrate host cells

• resistant strains and chemical treatments can reduce infection risk but no cure

black sigatoka - fungus

• attacks and destroys leaves

• hyphae penetrate leaves and turn them black

• can cause a 50% reduction in yield

• fungicide and good husbandry can control its spread, resistant strains are being developed but no cure

TB (tuberculosis) - bacteria

• affects humans, cows, pigs, badgers and deer

• destroys lung tissue and suppresses immune system

• can be cured by antibiotics and preventable through vaccination and improvement of living standards

meningitis - bacteria

• affects the meninges of the brain potentially leading to septicaemia and rapid death

• predominant in teenagers aged 15-19

• symptoms include a red/purple rash

• antibiotics and vaccines can cure the disease

HIV (human immunodeficiency virus) - virus

• targets T helper cells destroying the immune system making those affected susceptible to other diseases

• can affect humans and some non-human primates

• genetic material is RNA which is transcribed by enzyme reverse transcriptase producing a single strand of DNA

• can be passed through exchange of bodily fluids, unprotected sex, shared needles, contaminated blood products and from mothers to babies during pregnancy, birth or breastfeeding

• no vaccine, no cure but anti-retroviral drugs can be provided

influenza (flu) - virus

• affects the ciliated epithelial cells in the gas exchange system

• can be fatal

• affects humans, pigs and birds

• vaccines but no cure

• mutate regularly into new variants or strains

malaria (protist)

• spread by the vector - infected female Anopheles mosquitoes

• plasmodium protist reproduces in mosquito and mosquito bites human

• invades red blood cells, liver and even the brain

• no vaccine, preventative measures can be taken eg insecticides or mosquito nets but no cure

ring worm - fungus

• affects cattle, dogs, cats and humans

• causes grey white, crusty circular areas of skin

• antifungal creams are an effective cure

athlete’s foot - fungus

• digests the warm, moist skin between the toes

• cause cracking and scaling, itchiness and soreness

• antifungal creams can cure it

physical plant defences

• waxy cuticle

• bark

• cellulose cell walls

• callose production - a polysaccaride with beta 1,3 and 1,6 linkages is synthesised and deposited between the cell wall and cell membrane in cells near the infected cell which then acts as a barrier preventing the pathogen from entering, blocks sieve plates in phloem and is also deposited in the plasmodesmata between infected cells

• addition of lignin near the callose - makes the barrier stronger and thicker

chemical plant defences

• insect repellents

• insecticides

• antibiotics or antibacterial compunds

• antifungal compunds

• anti-oomycetes

• general toxins

direct transmission in animals

• direct contact

• inoculation

• ingestion

indirect transmission in animals

• fomites e.g bedding, clothing

• inhalation

• vectors e.g water, pets

direct transmission in plants

• contact between a healthy plant and a diseased plant

indirect transmission in plants

• soil contamination

• vectors e.g water, wind, animals or humans

factors affecting transmission in animals

• poor living and working conditions

• poor nutrition

• weak immune system

• poor waste disposal

• climate change

• culture and infrastructure

• socioeconomic factors

factors affecting transmission in plants

• planting varieties of crops

• proximity of crops

• poor mineral nutrition

• damp, warm conditions

• climate change

non specific defences in animals (keeping pathogens out)

• skin and sebum (which inhibits the growth of pathogens)

• mucus produced by goblet cells

• lysozymes

• stomach acid

process of blood clotting

1. when collagen is exposed to blood, clotting factors activate and serotonin causes smooth muscles in the walls of blood vessels to contract, reducing blood flow to the area.

2. thromboplastin (a clotting factor) and calcium ions catalyse prothrombin to thrombin which then catalyses fibrinogen to fibrin.

3. fibrin then glues together platelets and other clotting factors to form the blood clot.

4. the blood clot dries out and epidermal cells grow underneath and blood vessels regenerate. collagen fibres are deposited to strengthen new tissue.

5. the scab then sloughs off when new epidermis is thick enough

what is the inflammatory response?

mast cells are activated releasing chemicals like histamines and cytokines.

• histamines: make the blood vessels dilate, raise temperature and cause redness and forces out blood plasma forming tissue fluid which causes swelling (oedema) and pain

• cytokines: which attract phagocytes to the site are cell-signalling molecules that can also increase body temp and stimulate the specific immune system

non specific defences in animals (getting rid of pathogens)

• fevers - the hypothalamus resets the thermostat which is useful because higher temperatures inhibit pathogen reproduction and the specific immune system works faster

how does phagocytosis work?

1. pathogens produce chemicals that attract phagocytes e.g cytokines or opsonins e.g IgG or IgM (immunoglobulin G/M)

2. phagocytes recognise non-human proteins (antigens) on the pathogen

3. phagocyte engulfs the pathogen and encloses it in a vacuole called a phagosome

4. the phagosome combined with a lysosome form a phagolysosome

5. lysozymes from the lysosome digest and destroy the pathogen

6. macrophages (a differentiated phagocyte) combine antigens from the pathogen along with glycoproteins called major histocompatibility complexes (MHCs) to become an antigen-presenting cell.

what is the specific immune sytem?

it is an immune system that offers active/acquired immunity by ‘remembering’ an antigen after an initial response leading to an enhanced response to subsequent encounters.

structure and function of antibodies

• y-shaped glycoproteins called immunoglobulins that bind to antigens on a pathogen or toxin

• made up of two identical long polypeptide chains called heavy chains and two shorter chains called light chains which are held together by disulfide bridges

• antibodies bind to antigen similar to the ‘lock-and-key’ mechanism to form an antigen-antibody complex

• its binding site has an area of 110 amino acids and is known as the varibale region

• the hinge regions provide the antibody with flexibility

roles of antibodies?

• opsonin which attracts phagocytes

• barrier to pathogen entering once they are part of an antibody-antigen complex

• agglutinins causing the antibody-antigen complexes to clump together, so phagocytes can engulf them all at once

where do B lymphocytes mature?

bone marrow

where do T lymphocytes mature?

thymus gland

what are B effector cells?

the cells that divide to form plasma cells

what are plasma cells?

they produce the antibodies complementary to a specific antigen

what are B memory cells?

they are the cells that stay in the blood and ‘remember’ that antigen to initiate a rapid response when exposed to the antigen in the future again

what is humoral immunity?

this is when the body responds to antigen by producing antibodies to target pathogens

how does humoral immunity work?

1. a B cell with the complementary antibodies to a specific antigen engulfs and processes the antigens to become an APC (antigen presenting cell)

2. Activated T helper cells bind to the B cell APC (clonal selection - where a B cell with right antibodies is selected for cloning)

3. Interleukins produced by the T helper cells activate the B cells.

4. The activated B cell divides by mitosis to give clones of plasma cells and B memory cells (clonal expansion)

5. primary immune response - the plasma cells with the complementary antibodies bind to the antigens disabling them (slow)

6. secondary immune response - the B memory cells divide to form plasma cell clones which disable the antigens by producing antibodies complementary to the antigen (rapid)

what is cell-mediated immunity?

this is when T lymphocytes respond to the cells of an organism that have been changed by a pathogen

how does cell-mediated immunity work?

1. macrophages engulf and digest pathogens in phagocytosis and process the antigens to become APCs

2. the receptors on T helper cells fit antigens which cause them to become activated and produce interleukins, stimulating more T cells to divide rapidly by mitosis (clonal expansion) which can all differentiate into different types of T cells.

3. the cloned T cells can be: T memory cells, T regulator cells, T killer cells or T helper cells

4. the cloned T cells can produce interleukins to stimulate B cells or phagocytosis

what are T helper cells?

cells that produce interleukins which stimulate B and T cells and macrophages to ingest pathogens

what are T killer cells?

cells that produce perforin which destroys pathogens

what are T memory cells?

they form part of the immunological memory, dividing rapidly to form clones of T killer cells that can destroy the pathogen

what are T regulator cells?

these suppress the immune system, makes sure that the body recognises self-antigens and does not set up an autoimmune response

what is an autoimmune disease?

when the immune system stops recognising self cells and starts to attack healthy body tissue

how does type 1 diabetes affect the body and how can it be treated?

affects the insulin secreting cells of the pancreas, can be treated through insulin injections, pancreas transplants and immunosuppressant drugs

how does rheumatoid arthritis affect the body and how can it be treated?

affects the joints, can be treated with pain relief, immunosuppressants, steroids, and other drugs but no cure

how does lupus affect the body and how can it be treated?

affects the skin and joints, causing fatigue and can attack any organ in the body e.g liver, kidneys, brain and can be treated with immunosuppressants, steroids, anti-inflammatory drugs but no cure

natural active immunity

when you make your own antibodies in response to a pathogen that has naturally entered your body

natural passive immunity

when you receive antibodies from an individual naturally e.g through colostrum from the mother to babies

artificial passive immunity

when you receive antibodies from an individual through injections (artificially) which offers temporary immune protection e.g injections for rabies or tetanus

artificial active immunity

when you make your own antibodies in response to a pathogen that has artificially entered your body e.g vaccinations

what could vaccines contain?

• killed or inactivated pathogens

• attenuated strains of pathogens

• altered or detoxified toxin molecules

• isolated antigens from a pathogen

• genetically engineered antigens

what is herd immunity?

when a significant number of people in the population have been vaccinated, this gives protection to those who do not have immunity.

where does penicillin come from and what does it do?

from mould growing on melons and used as an antibiotic

where does docetaxel come from and what does it do?

from yew trees and used to treat breast cancer

where does aspirin come from and what does it do?

from willow bark and used as a painkiller

where does prialt come from and what does it do?

from cone snail venom and used as a painkiller (1000x more effective than morphine)

where does vancomycin come from and what does it do?

from a soil fungus and used as an antibiotic

where does digoxin come from and what does it do?

from foxgloves and used to treat heart problems

how can drugs be designed for the future?

• pharmacogenetics

• synthetic biology

what is selective toxicity?

when antibiotics interfere with the metabolism of bacteria without affecting the metabolism of human cells

what is MRSA and what does it do?

• bacteria

• causes boils, abscesses and potentially fatal septicaemia

• was treated with methicillin but that has also become resistant

what is C.difficile and what does it do?

• bacteria

• produces toxins that damages intestine lining, causes diarrhoea, bleeding or even death

• results when gut bacteria is killed off as a result of too much antibiotics being taken

how can antibiotic-resistant infections be reduced?

• minimising use of antibiotics

• good hygiene in hospitals, care homes, etc.